Power tools, including power drills and saws of various types, have enjoyed widespread use. Electric power tools in particular are very popular. The present invention is directed generally to means for providing motive power to tools and tools incorporating such motive power providing means. The invention will be discussed in terms of electrically-powered saws, but it should be emphasized that the claims, not the following description, are indicative of the extent of the invention.
Several different types of electrically-powered saws are available, including radial-arm saws, table saws, circular saws and power mitre boxes. A preferred embodiment of the invention includes a motor unit for a power mitre box, and the remaining discussion will focus on power mitre boxes by way of example. The present invention is not limited to power mitre boxes in any way, however.
As is well known, a power mitre box is essentially a mitre box and a circular saw, in combination. The saw is vertically hinged to a table which can horizontally swivel in relation to a base. The angle of the table to the base determines the angle of the cut. A power mitre box can be used by a carpenter to quickly and accurately cut molding, for example, for subsequent installation in a house.
Power mitre boxes usually include an electric motor which operates at approximately 20,000 revolutions per minute (RPM). A speed reducer assembly mechanically couples the armature of the electric motor to an arbor shaft which carries the saw blade. The speed reducer assembly, either gear or belt type, reduces the speed of the arbor shaft to approximately 4,000 to 5,000 RPM. The electric motor, the speed reducer assembly, the arbor shaft, and their related components constitute a "motor unit," and the present invention is primarily directed to such a motor unit.
At a minimum, power mitre boxes should be:
1. reliable; PA1 2. precise; PA1 3. safe; PA1 4. easy to repair; and PA1 5. capable of efficiently cutting through lumber having a reasonably large cross-section with a single pass. PA1 (i) the distance between the arbor bearings is at least approximately one-third the distance between the armature bearings; PA1 (ii) the distance between the arbor bearings is at least approximately one-half the tool diameter; or PA1 (iii) the length of the arbor shaft is at least approximately one-third the length of the armature. PA1 (i) a casing; PA1 (ii) an armature; PA1 (iii) a pair of armature bearings operatively connected to the casing for rotatably supporting the armature; PA1 (iv) an arbor shaft operatively coupled to the armature suitable for carrying the saw blade; and PA1 (v) a pair of arbor bearings operatively connected to the casing for rotatably supporting the arbor shaft, wherein the distance between the arbor bearings is at least approximately one-third the distance between the armature bearings.
Of the desirable characteristics listed above, the present invention is primarily directed toward improving the preciseness of power mitre boxes. In preferred embodiments, ease-of-repair and cutting capability are also enhanced.
As noted above, improving the accuracy or preciseness of a power mitre box is one aspect of the invention. A power mitre box can render a clean, precise, vertical cut only if it is supplied with a sharp blade which is securely connected to a horizontally-rigid arbor shaft. Play or radial runout in the arbor shaft can lead to an imprecise cut. Referring to FIG. 5, play in the arbor shaft can result in movement of the saw blade away from the vertical such that the power mitre box will not make a precise vertical cut. FIG. 5B diagrammatically illustrates a typical prior art arbor shaft supported by a pair of closely spaced ball bearings. Looseness in the bearings necessarily allows play in the arbor shaft, and this effect is quite pronounced due to the close spacing of the ball bearings.
In addition to the arbor shaft play problem discussed above, prior art power mitre boxes typically include speed reducer assemblies which tend to limit their cutting capability (factor No. 5 set out above). As mentioned above, there are two types of speed reducers, gear and belt. In the case of a gear speed reducer, a pinion gear is mounted on the armature shaft. It meshes with and drives a larger diameter gear carried by the arbor shaft. In belt reducers, a small sprocket is attached to the armature, a larger diameter sprocket is mounted on the arbor shaft and a cog belt interconnects the two sprockets. In either case, the diameter of the relatively larger gear or sprocket necessarily limits the depth of cut provided by a given saw blade. That is, the motor unit casing surrounding the larger gear or sprocket comes into contact with the workpiece before the arbor shaft, thus limiting full utilization of the saw blade. Prior art power mitre boxes have short arbor shafts which locate the larger sprocket or gear in close proximity to the blade. The 37 bulge" in the motor unit casing associated with the larger gear or sprocket therefore comes into play during every cut (of thick lumber), and the entire radius of the blade is not utilized.
While most prior art power mitre boxes (and other power saw devices, for that matter) are useful for their intended purposes, it is perceived that they are lacking in some areas. In particular, arbor shaft deflection is unnecessarily large. Also, saw blades are often underutilized, at least for mitre or bevel cuts. Finally, the speed reducer assemblies of some prior art motor units are expensive to repair. The present invention is primarily directed toward reducing arbor shaft and blade deflection; and preferred embodiments address the blade utilization and speed reducer repair problems associated with prior art devices.